Free Email Subject Line Generator (+ Guide)

What is an email subject line generator?

An email subject line generator writes subject lines for an email from what it's about and who it's for.

This one uses your own AI, so it costs nothing, and it gives your AI what won in real tests to work from.

Where the score comes from, and how far to trust it

I read 56,262 emails from 579 public newsletters on Substack's leaderboards, each with its subject line and how readers reacted. On Substack, a post's title is its email subject unless the writer changes it.

Then I compared emails from the same newsletter, sent to the same readers within 45 days. In pairs where one got at least twice the reactions, this is how often a subject with each part won:

A model trained on those pairs gives the score: where your line's wording lands among all 56,262 subject lines. On 107 newsletters it never saw, the line it scored higher won 56% of the time, when a coin flip gets 50%.

That's a real edge, but a small one. The topic matters more than the wording, and reactions come from people who opened and read, so they measure the whole email. Use the score to pick between lines you like, then test the top 2 on your list.

The emails are newsletters. Launch, promo, welcome and cold emails get the same score, and the checks count as much for them, until I have their own proven lines.

Spam: the subject is a small part

Gmail looks at the sender first. Its sender guidelines ask every sender for SPF or DKIM, a spam rate under 0.3%, and no fake "Re:" or "Fwd:". Bulk senders also need DMARC and one-click unsubscribe.

The subject adds a little on top. SpamAssassin, the open source spam filter, scores these subject habits, and calls an email spam at 5 points:

So a subject alone rarely sinks an email. A sender with a bad reputation sinks every subject.

How to test subject lines

Send line A to half your list and line B to the other half, and count clicks, not opens. Apple's Mail Privacy Protection hides whether people opened.

The A/B test calculator tells you how big each half needs to be, and when the winner is real.

Who is the email subject line generator for?

It's for anyone who writes emails people should open:

• Email subject line tester – people checking a line before sending
• Email countdown timer – anyone announcing a deadline
• A/B test calculator – marketers testing two lines
• Headline analyzer – writers working on headlines too

The rest of my tools are on the free tools page.